The Basics; The Vaccine, It Seems, Lacks Moxie

By DENISE GRADY

Published: April 23, 2006

An outbreak of mumps in the Midwest has made more than 1,300 people sick in Iowa and seven other states, the worst epidemic of the disease in the United States in 20 years.

Mumps, a viral infection, is usually not serious, and there have been no reports of deaths. The most troubling aspect of the epidemic is that many of the patients had been vaccinated. Does this mean that millions of people who thought they were safe actually are not?

There are no clear answers yet. But the outbreak has brought to light some unsettling facts about the vaccine, which is actually a combination of vaccines for measles, mumps and rubella, and is given in two shots, usually in early childhood.

Even among those given both shots, 10 percent are left unprotected against mumps. The failure rate for measles and rubella is only 1 percent. For those who skip the second shot, 20 percent have no mumps protection, with 6 percent to 8 percent remaining vulnerable to measles and rubella.

Health officials say those figures, combined with the fact that some people may not have been vaccinated at all, explain why so many have been susceptible.

The officials suggest that someone brought the virus to Iowa from England, which has had tens of thousands of cases. In Iowa, the virus quickly began spreading on college campuses. There may have been a delay in detecting the outbreak because most doctors and nurses today have never seen a case of mumps.

''This is a much larger outbreak than any of us anticipated,'' said Dr. William Schaffner, an infectious disease expert at Vanderbilt University in Nashville. He said it might be significant that most of the cases involved young adults, not children. ''Maybe the immunity generated by the mumps vaccine can wane and then you become newly susceptible, particularly if you encounter a large dose of virus,'' he said. DENISE GRADY